In the working environment of a coal mine, off-road trucks may be used to haul both coal and overburden. Overburden, which is the earth (rock and dirt) which must be removed in surface coal mining operations to expose the coal seams for mining, typically has a greater density than the coal being mined.
The difference in material density between coal and overburden has created problems for mine operators. When hauling the higher density overburden in heavy-duty off-road trucks having bodies with tailgate assembly sized to haul coal, the load may substantially exceed the allowable gross vehicle weight rating of the off-road truck if the coal-rated bodies with tailgate assembly are filled to their full volume capacity. In off-road trucks having overburden-rated truck bodies, the truck bodies are sized to minimize the possibility of overloading the truck when hauling overburden. The reduced volumetric capacity of the overburden-rated truck body, however, prevents the off-road truck from hauling its maximum allowable gross vehicle weight-rated capacity in lighter density coal. When the lighter weight coal is hauled, sometimes less than half of the allowable payload capacity of the off-road truck may be utilized.
Although some mine operators employ a fleet of off-road trucks comprising one group of off-road trucks equipped with the overburden-rated, smaller volume bodies and a second group of off-road trucks equipped with the coal-rated, larger volume truck bodies, the dedication of trucks to a particular type of hauling task in total reduces the versatility of a mine operator's fleet because, at different times during the mining cycle, there may be a need for more off-road trucks to haul overburden while, at other times, there may be a need for more off-road trucks to haul coal. Thus, the mine operator must face the unpleasant choice of either operating the off-road trucks inefficiently when hauling coal by using a smaller volume overburden-rated truck body or, alternatively, if a larger volume coal-rated truck body with tailgate assembly is utilized for hauling overburden, the truck body can easily, if fully loaded with heavier overburden, cause off-road truck overloading. Obviously, dedicating the off-road truck fleet to groups intended for hauling different types of materials reduces the flexibility of the fleet and increases the possibility of inefficient fleet operation.
A solution to the problem of reducing off-road truck fleet flexibility when needing different size truck bodies has been to selectively utilize a tailgate assembly with truck bodies in order to adjust the volumetric capacity of the truck body. In truck bodies designed for the dual purpose of hauling both coal and overburden, it becomes essential that a tailgate assembly be utilized due to body length and height constraints and the heaping nature of the coal being hauled. By utilizing a tailgate with the truck body, the volumetric capacity and the corresponding load capabilities of the truck body can be designed to the proper capacity for hauling coal, while the removal of the tailgate permits the volumetric capacity of the truck body to be reduced for hauling overburden.
Unfortunately, the addition of a tailgate to a truck body poses some potential disadvantages if used when hauling overburden on a truck body primarily designed for hauling coal. The tailgate can interfere with the loading operation of a truck if, when loading with a loading shovel, the loading shovel must raise the loading shovel bucket over the tailgate. This situation is justified for loading coal because of the additional load capacity provided by the tailgate. However, additional capacity is typically not needed with a coal/overburden truck body tailgate assembly combination when hauling overburden. And, when loading overburden with a loading shovel, the height of the tailgate creates a risk that overburden may be dropped into the truck body from a greater height.
One attempt to remedy the challenges created by selective use of a tailgate is described in U.S. Pat. No. 4,621,858 issued to LeRoy G. Hagenbuch on Nov. 11, 1986 which describes a tailgate rotatable between a closed, in-service position at the rear of the coal hauling truck bed for increasing the volumetric capacity of the truck body and an out-of-service position at the front of the truck body for reducing volumetric capacity of the truck body for use in hauling overburden and other high density material. In the out-of-service position, the tailgate does not affect the operation of the loading shovel. The means for rotating the tailgate between the in-service and out-of-service position utilizes the center of gravity of the tailgate relative to a vertical axis through the tailgate pivot point such that the tailgate rotates to the out-of-service position when the tailgate center of gravity is forward of the vertical axis of the tailgate pivot point and to the closed, in-service position when the tailgate center of gravity is rearward of the vertical axis of the tailgate pivot point.
Unfortunately, the tailgate assembly described in U.S. Pat. No. 4,621,858 has several disadvantages. The rotating of the tailgate between in-service and out-of-service position of the tailgate is adversely affected when the truck is operated on adverse grades because the position of the tailgate center of gravity with the truck body raised changes relative to the vertical axis through the tailgate pivot point depending on truck grade placement (parked up hill, parked down hill).
In addition, in rotating the tailgate assembly between the in-service closed and out-of-service positions, the rotation of the tailgate is uncontrolled for a brief part of its travel. During this brief period of uncontrolled tailgate rotation, gravity causes the tailgate to accelerate in the rotation, gathering some momentum, creating forces and stresses which can adversely affect the tailgate. Additionally, this method of tailgate rotation requires hooking, unhooking, and reeving of the tailgate actuation chain by either the truck operator leaving the truck cab or a second individual on the ground--something that both raises hazards and complicates the process of moving the tailgate from the in-service to out-of-service positions.